Organocatalytic Transfer Hydrogenation and Hydrosilylation Reactions
The reduction of different carbon–carbon or carbon–heteroatom double bonds is a powerful tool that generates in many cases new stereogenic centers. In the last decade, the organocatalytic version of these transformations has attracted more attention, and
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Organocatalytic Transfer Hydrogenation and Hydrosilylation Reactions Raquel P. Herrera1
Received: 16 December 2015 / Accepted: 26 April 2016 / Published online: 7 May 2016 Springer International Publishing Switzerland 2016
Abstract The reduction of different carbon–carbon or carbon–heteroatom double bonds is a powerful tool that generates in many cases new stereogenic centers. In the last decade, the organocatalytic version of these transformations has attracted more attention, and remarkable progress has been made in this way. Organocatalysts such as chiral Brønsted acids, thioureas, chiral secondary amines or Lewis bases have been successfully used for this purpose. In this context, this chapter will cover pioneering and seminal examples using Hantzsch dihydropyridines 1 and trichlorosilane 2 as reducing agents. More recent examples will be also cited in order to cover as much as possible the complete research in this field. Keywords Transfer hydrogenation Organocatalysis Hantzsch ester Trichlorosilane Phosphoric acid Aminocatalysis Thioureas Lewis bases Reduction Hydrosilylation
1 Introduction The reduction of different carbon–carbon or carbon–heteroatom double bonds is an important transformation that generates in many cases new stereogenic centers. Particularly, the asymmetric reduction of prochiral ketimines represents one of the most important methods and straightforward procedures for preparing chiral amines. This approach is one of the key reactions and powerful tools in synthetic organic This article is part of the Topical Collection ‘‘Hydrogen Transfer Reactions’’; edited by Gabriela Guillena, Diego J. Ramo´n. & Raquel P. Herrera [email protected] 1
Laboratorio de Organocata´lisis Asime´trica. Instituto de Sintesis Quı´mica y Cata´lisis Homoge´nea (ISQCH-CSIC), Saragossa, Spain
Reprinted from the journal
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Fig. 1 Model reducing agents
chemistry, which provides precious building blocks for natural products, pharmaceutical, and other fine chemical industries [1]. Until the last decade, available chemical catalysts for the enantioselective reduction of these substrates were mostly limited to chiral transition metal complexes, which often required elevated pressures and/or the use of additional additives to afford high yields and ee values (for reviews, see [2–10]). However, with the increasing interest during the last years in the development of the organocatalysis field [11–13], the organocatalytic version of these transformations has attracted more attention, and remarkable progress has been made in this way (For selected reviews on organocatalytic transfer hydrogenations, see [14–20]). The organocatalytic transfer hydrogenation is carried out by four fundamentally different approaches (Fig. 1): (1) reduction with Hantzsch dihydropyridines 1, mainly catalyzed by chiral Brønsted acids, which activate the electrophilic substrates (for reviews, see: [21–26]); (2) hydrosilylation with trichlorosilane 2, catalyzed through chiral Lewis-bases, wh
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